1. Field of the Invention
The present invention relates, in general, to spacer grids used for placing and supporting fuel rods in nuclear reactor fuel assemblies and, more particularly, to a spacer grid with grid springs designed to have an optimal shape of their vertical support parts, thus reducing fretting wear of the fuel rods caused by contact of the fuel rods with the grid springs, the grid springs also designed to optimize the distribution and to minimize the intensity of contact stress caused by the contact between the springs and the fuel rods, thus being less likely to cause fretting wear of the fuel rods and stably placing and supporting the fuel rods in the assembly during the effective life of the fuel rods expires, and thereby improving the soundness of the assembly.
2. Description of the Prior Art
As well known to those skilled in the art, spacer grids are elements of a nuclear reactor fuel assembly, and each has a plurality of grid springs and dimples in their fuel rod cells for placing and supporting a plurality of fuel rods within the spacer grids of the fuel assembly.
FIG. 1 is an exploded perspective view, showing the construction of a typical nuclear reactor fuel assembly. FIG. 2a is a plan view, showing a conventional spacer grid used in the fuel assembly of FIG. 1.
FIG. 2b is a side view of the spacer grid of FIG. 2a, with one fuel rod placed and supported by grid springs and dimples within a fuel rod cell of the spacer grid of FIG. 2a. FIG. 3 is a sectional view, showing a deformation of a conventional grid spring due to a force applied from a fuel rod set in the cell of the spacer grid of FIG. 2a. 
FIG. 4a is a perspective view of a conventional grid spring used in the prior art spacer grid. FIG. 4b is a perspective view of a conventional dimple used in the prior art spacer grid.
In a typical nuclear reactor fuel assembly 2 having a plurality of spacer grids 110, a plurality of guide tubes 113 are vertically arranged between top and bottom support pallets 111 and 112.
The spacer grids 110 for placing and supporting the fuel rods 125 in the assembly 2 are arranged along the guide tubes 113 at regular intervals in a vertical direction, and fixed to the tubes 113 through a welding process.
Each of the spacer grids 110 is made of a zircaloy or Inconel, and defines both a plurality of fuel rod cells 123 for supporting the fuel rods 125 and a plurality of guide tube cells 124 for supporting the guide tubes 113. Each of the fuel rod cells 123 has two grid springs 118 and four dimples 119, and so the fuel rod 125 inside each fuel rod cell 123 is supported at six positions through point contact-type support and surface contact-type support. The four dimples 119 are arranged in each fuel rod cell 123 such that two dimples 119 are formed at positions above and under each grid spring 118.
Since the grid springs 118 of the spacer grid 110 are brought into direct contact with the fuel rods 125, the springs 118 may be deformed by the fuel rods 125. That is, the vertical support parts 121 or the root parts of a grid spring 118 may be depressed due to the force applied from a fuel rod 125 to the grid spring 118 as shown in FIG. 3. In such a case, as the strength of the vertical support parts 121 of each grid spring 118 is almost equal to that of the central curved support part 122 of the spring 118 at which the spring 118 comes into direct contact with the fuel rod 125, the vertical support parts 121 of the spring 118 are deformed by both a bending moment 131 and a twisting moment 132 caused by the force applied from the fuel rod 125 to the spring 118 at the same time. Such simultaneous application of the bending moment 131 and twisting moment 132 to the vertical support parts 121 of the springs 118 prevents the springs 118 from stably supporting the fuel rods 125, in addition to making the springs 118 fail to effectively resist fatigue due to stress. In addition, the external surface of the fuel rod 125 may slide slightly on the central curved support part 122 of the spring 118 in the case of the application of the bending moment and twisting moment to the spring 118. In such a case, a fretting wear of the fuel rods 125 inside the spacer grid 110 may be easily caused by sliding movement.
In the spacer grid 110, the grid springs 118 and the dimples 119 of each fuel rod cell 123 have the same radius of curvature as that of the fuel rods 125 to accomplish conformal contact of the springs 118 and dimples 119 with the fuel rod 125. However, the strength of the vertical support parts 121 of each spring 118 is almost equal to that of the central curved support part 122 of the spring 118 as described above, and so the two parts 121 and 122 are deformed at the same time when a force is applied from the rods 125 to the spring 118.
Therefore, the external surface of the fuel rod 125 slides slightly on the central curved support part 122 of the spring 118, while the vertical support parts 121 of the spring 118 is deformed by the bending moment and twisting moment applied from the fuel rod 125 to the spring 118. Therefore, the vertical support parts 121 are bent in a loaded direction in addition to being distorted.
When the grid springs 118 and dimples 119 of the conventional spacer grids 110 have insufficient spring force, it is almost impossible for them to stably place and support the fuel rods 125 at desired positions inside the assembly 2, thus reducing the soundness of the fuel assemblies. On the contrary, when the grid springs 118 and dimples 119 of the conventional spacer grid 110 are too high in their spring forces, there may be formed excessive frictional force between the fuel rods 125 and the spacer grids 110 during an insertion of the fuel rods 125 into the cells 123 of the grids 110. Such an excessive frictional force may cause damages, such as scratches, on the external surface of the fuel rods 125, and fail to appropriately support the fuel rods 125 in the case of lengthwise growth of the fuel rods due to neutron radiation during the operation of a nuclear reactor. In such a case, the fuel rods 125 may be undesirably bent.
When the fuel rods 125 bend as described above, the rods 125 become closer to each other to be sometimes brought into undesired contact with each other, thus making the coolant channels between the fuel rods 125 of the fuel assembly 2 become narrower or closed. In such a case, it could impede the effective heat transfer from the fuel rods 125 to the coolant, and so the fuel rods 125 may be partially overheated, thus sometimes causing a DNB (Departure from Nucleated Boiling) and reducing the output power of the nuclear fuel.
The recent trend of development in the nuclear reactor fuel assemblies aims at the provision of high burn-up and defect-free nuclear fuel.
Particularly, in order to provide desired high burn-up nuclear fuel, it is necessary to improve the heat transfer efficiency between the fuel rods and the coolant in the nuclear reactor fuel assembly. The above object may be accomplished by designing the nuclear reactor fuel assembly to allow the coolant to enhance the mixing flow around the fuel rods in the assembly.
In an effort to accomplish such mixing flow of coolant within the nuclear reactor fuel assembly, several types of spacer grids having new structures have been proposed. For example, the mixing flow of the coolant may be accomplished by attaching specifically designed mixing vanes in the fuel assembly or providing effective coolant channels in the assembly.
The conventional techniques for mixing flow of coolant in the nuclear reactor fuel assemblies are based on the formation of more active turbulent flow of coolant with high Reynolds number around the fuel rods of the assembly. Therefore, the conventional techniques undesirably induce the fuel rod vibration in the assembly due to the active turbulent flow of coolant. Such vibration of the fuel rods in the nuclear fuel assemblies is a so-called xe2x80x9cflow-induced vibrationxe2x80x9d.
The flow induced vibration of the fuel rods in the nuclear reactor fuel assembly makes the fuel rods slide or move relative to the grid springs and dimples at their contact surfaces, thus partially rubbing the contact surfaces of the fuel rods due to frictional force. The flow-induced vibration of the fuel rods finally causes a fretting wear of the fuel rods.
The technique for improving the thermal efficiency of the nuclear fuel assembly and providing highly combustible nuclear fuel undesirably damages the fuel rods.
While designing the spacer grids for nuclear fuel assemblies, it is necessary to accomplish the following two requirements. That is, the spacer grids must stably support the fuel rods during the effective life of the fuel rods, in addition to being free from causing a fretting wear of the fuel rods.
First, in order to allow the spacer grids to stably support the fuel rods within a nuclear reactor fuel assembly during the effective life of the fuel rods, the spacer grids must be designed such that they effectively support the fuel rods with sufficient spring force of their grid springs and dimples. In addition, it is necessary to enlarge the elastic range of the grid springs and dimples, thus maintaining a desired spring force regardless of variable fuel rod support conditions inside the fuel assembly during the effective life of the fuel rods.
However, the grid springs and dimples of a conventional spacer grid for nuclear fuel assemblies gradually lose their original spring forces due to a neutron radiation during an operation of a nuclear reactor. Therefore, the grid springs and dimples may fail to desirably support the fuel rods, and there may be formed gaps between them. Due to such gaps, the spacer grids do not stably support the fuel rods, but undesirably allow the fuel rods to move by the flow of coolant. The spacer grids thus reduce the soundness of the nuclear reactor fuel assemblies.
Second, the protection of the fuel rods from fretting wear in a nuclear reactor fuel assembly may be accomplished by removing the causes of such fretting wear.
The causes of such fretting wear of fuel rods in a nuclear reactor fuel assembly include gaps formed between the grid springs, dimples and fuel rods. The gaps may be formed by a reduction in the spring force of the grid springs and dimples due to neutron radiation, a difference in the thermal expansion between the fuel rods and spacer grids, and a reduction in the diameter of the fuel rods caused by a lengthwise growth of the fuel rods. When such gaps are formed between the grid springs, dimples and fuel rods, the fuel rods are repeatedly brought into contact with and removed from the grid springs and dimples due to axial and transversal flow of coolant, and thus occurring feasibility of the fretting wear is increased.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a spacer grid for nuclear reactor fuel assemblies, which has a plurality of grid springs designed to have an optimal shape of their vertical support parts, thus allowing the vertical support parts of the grid springs to be deformed only by a bending moment in the case of an application of a force from the fuel rods to the springs, and which thus continuously maintains the desired conformal surface contact between the grid springs, dimples and the fuel rods regardless of any directional movement of the fuel rods within the fuel assembly, and reduces the fretting wear of the fuel rods caused by abnormal contact between the grid springs and fuel rods.
Another object of the present invention is to provide a spacer grid for nuclear reactor fuel assemblies, of which the grid springs have an optimal shape suitable for maintaining the desired conformal surface contact between the grid springs and the fuel rods during an operation of the fuel assembly, in addition to optimize the distribution and to minimize the intensity of contact stress caused by the contact between the grid springs and the fuel rods, thus effectively reducing the fretting wear of the fuel rods caused by abnormal contact between the grid springs and fuel rods.
A further object of the present invention is to provide a spacer grid for nuclear reactor fuel assemblies, of which the grid springs have an optimal shape of their vertical support parts, such that the springs enlarge their elastic range in a spring characteristic diagram or a force-displacement diagram, and which thus maintains the spring force capable of effectively supporting the fuel rod within the elastic limit of the springs, thus almost completely preventing an excessive plastic deformation of the grid springs during an insertion of the fuel rods into the spacer grids, in addition to stably placing and supporting the fuel rods in the fuel assembly during the effective life of the fuel rods, irrespective of variable fuel rod support conditions of the fuel assembly during an operation of a nuclear reactor.
In order to accomplish the above objects, the present invention provides a spacer grid for placing and supporting a plurality of longitudinal fuel rods in a nuclear reactor fuel assembly, comprising a plurality of inner strips arranged while intersecting each other at right angles prior to being encircled with four perimeter strips, thus forming an egg-crate pattern, the inner and perimeter strips each being fabricated with a plurality of unit strips arranged in parallel, and forming a plurality of four-walled fuel rod cells for receiving and supporting the fuel rods therein while isolating the fuel rods from each other, each unit strip of the inner strips having one grid spring formed on a central portion of the unit strip while projecting in a direction, and two dimples formed on the unit strip at positions above and under the grid spring while projecting in a direction opposite to the grid spring, and each unit strip of the perimeter strips having one grid spring formed on a central portion of the unit strip while projecting in a direction, wherein the grid spring comprises: a vertical support part including: an opening formed at the central portion of the unit strip; upper and lower base parts extending downward and upward from central portions of top and bottom edges of the opening; and two bridge parts branched from the upper base part and extending symmetrically downward until they are united at the lower base part; and a fuel rod support part including: a conformal support part brought into surface contact with an external surface of a fuel rod; and two transverse connection parts extending outward from opposite outside edges of the conformal support part while being specifically bent, and integrated with the central portions of inside edges of the two bridge parts into a single structure, thus connecting the fuel rod support part to the vertical support part.
In the spacer grid, the vertical support part has a plurality of bent portions and projects from the unit strip toward the fuel rod, thus elastically supporting load applied from the fuel rod thereto through the conformal support part 45 coming into direct contact with the fuel rod.
The conformal support part has the same radius of curvature as that of the fuel rod, thus being brought into surface contact with the external surface of the fuel rod, the conformal support part also having a circular or elliptical profile at its contact surface, thus enlarging a surface contact area thereof relative to the fuel rod, in addition to accomplishing a uniform contact pressure distribution and reducing a peak stress thereof.
The vertical support part is shaped through a bending process, and so its strength is relatively lower than that of the fuel rod support part having the conformal support part, the vertical support part being thus elastically deformed prior to a deformation of the conformal support part so as to allow the conformal support part to maintain a conformal contact with the fuel rod in the case of an application of a force from the fuel rod to the grid spring.
The vertical support part, including the upper and lower base parts and two bridge parts, further includes a plurality of bent portions formed through a bending process, thus being mainly affected by a bending moment, with a twisting moment-induced deformation of the vertical support part being minimized.
The vertical support part is bent at a plurality of portions through a bending process, thus having an enlarged elastic range and elastically supporting the fuel rod regardless of a variation in fuel rod support conditions in the nuclear reactor, and maintaining a spring force capable of effectively supporting the fuel rod within the elastic limit of the grid spring.
In the spacer grid, the base parts and bridge parts of the vertical support part are changeable in their widths and positions of their bent portions in accordance with design objects, thus providing optimized spring characteristic curves.
In addition, the dimples of the spacer grid have the same radius of curvature as that of the fuel rods, thus having curved contact surfaces capable of coming into conformal surface contact with the external surfaces of the fuel rods.
The dimples and conformal support parts of the grid springs are rounded or curved at their upper and lower edges in a direction opposite to the contact surfaces thereof with the fuel rods.
In another embodiment of the present invention, the grid spring of the present invention have upper and lower opening surrounded by the vertical support part and the fuel rod support part, and further include upper and lower extending part each extending from center of upper/lower base part of the vertical support part and partially enclosing said upper/lower opening.